1. Field of the Invention
The present invention relates to a focusing screen used in a single-lens reflex camera or the like, and a method of manufacturing the focusing screen.
2. Related Background Art
A conventional focusing screen for a single-lens reflex camera or the like is manufactured by a method of grinding the surface of a base material such as glass to form micropatterned projections, forming a mold from the resultant master, and transferring the micropatterned projections on the surface of a plastic material. This focusing screen has an advantage in an natural defocus effect, but has disadvantages in that the finder is dark and the coarse texture is conspicuous because the micropatterned projections on the surface of the focusing screen are formed highly at random.
Another conventional focusing screen is proposed in Japanese Patent Laid-Open Application No. 55-90931 wherein the random pattern is eliminated by cyclically arranging microlenses.
FIGS. 1A, 1B, and 1C are views for explaining a focusing screen of this type. FIG. 1A shows a cyclic array of microlenses 2 each having a predetermined height h. A central position (x,y) of the microlens 2 is defined by coordinates determined by a pitch p and a pair of integers (i,j) as follows: EQU x=p.times.(i+j/2) Eq. 1-1 EQU y=p.times.(j.times..sqroot.3/2) Eq. 1-2
Since the diffusion distribution is preferably isotropic, each microlens 2 preferably has a circular circumferential shape. The above arrangement represents an array of circles at a maximum density in a two-dimensional space. A sectional view of the array taken along the x-axis is shown in FIG. 1B. FIG. 1C shows an array of bright points of the diffusion distribution obtained when the array of the microlenses 2 is given as shown in FIG. 1A. Angular coordinate values (.theta..sub.x,.theta..sub.y) of the center of each bright point are determined by an angular pitch .theta..sub.p and a pair (i,j) of integers as follows: EQU .theta..sub.x .theta..sub.p .times.(i) Eq. 2-1 EQU .theta..sub.y =.theta..sub.p .times.(-i/.sqroot.3+2.times.j/.sqroot.3) Eq. 2-2
The pitches p of the microlenses 2 and the angular pitches .theta..sub.p of the bright point array of the diffusion distribution satisfy the following relation: EQU .theta..sub.p =.lambda./p Eq. 3
Thus, the angular pitch .theta..sub.p is proportional to a reciprocal of the pitch p. In the above equation, .lambda. represents the wavelength of light.
The focusing screen having the above arrangement is free from the coarse texture and is bright. However, this focusing screen has a disadvantage in an unnatural defocus phenomenon such as multi-line defocusing due to the following reason. Since this focusing screen has a cyclic structure, it has the same function as that of a diffraction grating. Diffused light is limited to a specific direction corresponding to the orders of diffraction.
Since the above diffraction effect is caused by the cyclic microlens array, a method of providing an appropriate random pattern to the cyclic microlens array is proposed in Japanese Patent Laid-Open Application No. 63-221329. In order to eliminate the unnatural defocus phenomenon according to this method, a considerably conspicuous random pattern is required. When an extremely conspicuous random pattern is used, the density of the microlens array becomes nonuniform. As a result, narrow-angled diffusion light (i.e., diffusion light close to light directly passing through the lens without any diffraction) is increased. As a result, it is difficult to perform a focusing operation.
A diffraction effect caused by the cyclic microlens array will be described in detail below. The diffusion light is limited to a direction corresponding to the degree of diffraction in accordance with the diffraction effect, as described above. These conditions are shown in FIGS. 2A, 2B, and 2C. As shown in FIG. 2A, assume that microlenses 2 are cyclically arranged at pitches p (the sectional view along the x-axis is shown in FIG. 2B). Diffusion light is cyclically arranged as bright points 3 at pitches .theta..sub.p in accordance with the diffraction effect, as shown in FIG. 2C. FIG. 2C shows an angular space, in which the pitches p of the microlenses 2 and the angular pitches .theta..sub.p of the bright point array of the diffusion distribution satisfy the following relation: EQU .theta..sub.p =.lambda./p Eq. 4
where .lambda. is the wavelength of light. When the defocus state of a point light source is observed through a focusing screen having such a diffusion distribution, a defocus image is also observed as an aggregate of bright points. Since a linear object is regarded as an object in which point light sources are linearly aligned, a defocus image of the linear object is an array of defocus images of point light sources. As a result, a so-called multi-line defocus state occurs. Since an arbitrary object is regarded as an aggregate of point light sources, a defocus image of the arbitrary object becomes unnatural accordingly. When bright points 3 in FIG. 2C are numbered, and x and y angular coordinates of the center of the ith bright point are defined as .theta..sub.xi and .theta..sub.yi, x and y coordinates xi and yi of the center of the bright point of the defocus image of the point light source corresponding to this bright point 3 are given as follows: EQU xi=.beta...DELTA.d..theta..sub.xi Eq. 5-1 EQU yi=.beta...DELTA.d..theta..sub.yi Eq. 5-2
where .beta. is the magnification of an observation system, and .DELTA.d is the defocus amount. If the defocus amount .DELTA.d=0, i.e., if an in-focus state is obtained, xi=yi=0 for all bright points i. This indicates that an image of a point light source is observed as one point. Eqs. 5-1 and 5-2 represent that a defocus image of a point object is similar to the diffusion distribution of FIG. 1C. When a distance between bright points in the diffusion distribution is small, a distance between the bright points of the defocus image is small. When the distance between the bright points is small, the number of bright points is increased, and the brightness of each bright point is decreased in inverse proportion to the increase in the number of bright points. As a result, the bright points are less conspicuous. The defocus state becomes more natural when the bright points are less conspicuous.
The distance .theta..sub.p between the bright points in the diffusion distribution is proportional to a reciprocal of the array pitch p of the microlenses, as indicated by Eq. 4. When the pitch is increased, the distance between the bright points in the diffusion distribution is decreased. The distance between the bright points of the defocus image is decreased, and the unnatural defocus phenomenon becomes less conspicuous. An increase in array pitch indicates an increase in size of each individual lens. When the size of this microlens is extremely increased, the microlens itself is observable to a human eye. When the size of the microlens is generally increased, narrow-angled diffusion light is increased. It is, therefore, difficult to perform a focusing operation. This is the problem solved by the first aspect of the present invention (to be described later).
As a technique of manufacturing a focusing screen, there is a method in which a focusing screen master having micropatterned projections on its surface is prepared, a mold is formed from this master, and the micropatterned projections are transferred to the surface of a plastic material.
As also described before, according to the conventional method of manufacturing the focusing screen master, the surface of the master is ground to form micropatterned projections. The focusing screen obtained from the focusing screen master manufactured from this method has an advantage in a natural defocus effect, but has disadvantages in that the frame is dark and the coarse texture is observable due to highly random micropatterned projections on the surface of the focusing screen. In order to eliminate these disadvantages, regular or semi-regular micropatterned projections (appropriate random pattern) may be formed on the surface of the focusing screen, and various methods have been proposed to achieve this. As one of these methods, a mask base having a micropattern is used and comes close to the surface of a photosensitive material applied to the surface of a substrate, and the substrate is exposed to prepare a focusing screen, the surface of which has micropatterned projections corresponding to an exposure amount, as proposed in Japanese Patent Laid-Open Application Nos. 57-148728 and 63-221329. According to this method, a so-called proximity exposure method used in the manufacture of semiconductor devices and the like is directly utilized in the manufacture of focusing screens. A mask 40 having a micropattern 30 formed on a mask base 20 shown in the plan view of FIG. 3 is spaced apart from a substrate 60 applied with a photosensitive material 50 on its surface as shown in the sectional view of FIG. 4 by a distance .DELTA.d (.DELTA.d is called proximity), and the photosensitive material 50 is exposed with an exposure beam 70. An illuminance distribution determined by the micropattern 30 of the mask base 20 and the proximity (.DELTA.d) is formed on the surface of the photosensitive material 50. After the photosensitive material 50 is exposed and developed, a micropatterned projection surface of the photosensitive material 50 is formed in correspondence with the illuminance distribution, thereby preparing a focusing screen master.
A focusing screen manufactured from this focusing screen master has a surface of regular or appropriately random micropatterned projections. Therefore, this focusing screen has a sufficiently high brightness level and a finer texture.
Although an array of regular micropatterned projections described in Japanese Patent Laid-Open No. 57-148728 has advantages in that a finer texture can be obtained and the frame is bright, an unnatural defocus phenomenon such as a multi-line defocus phenomenon is caused because the diffusion distribution is an aggregate of bright points 10, as shown in FIG. 5. In Japanese Patent Laid-Open Application No. 63-221329 for providing a random pattern to the array of micropatterned projections so as to solve the unnatural defocus phenomenon, when a weak random pattern is employed, bright points of lower orders become conspicuous. When a strong random pattern, however, is employed, the coarse texture is observable, resulting in inconvenience. These problems are solved by the third aspect of the present invention.